Method and bath for electrodeposition of aluminum



METHOD AND BATH F OR ELECTRODEPOSITION OF ALUMINUM t Arthur C. Doumas, Lake Jackson, Tex., and Nelson Murphy, Biacksburg, Va., assignors to Research Corporation, New York, N.Y., a corporation of New York No Drawing. Application August 23, 1956 Serial No. 605,713

9 Claims. (Cl. 204-14) This invention relates to the electrodeposition of aluminum, and particularly to a process involving the deposition of aluminum on a suitable cathode and to a new and improved electrolytic bath therefor.

Aluminum electrodeposited on a base metalimp-arts the qualities of good appearance and high resistance to corrosion. Because of the excellent corrosion'resistance of aluminum, protective coatings thereof on steel and other metals which are more easily corroded have been found to be highly desirable. In addition to the excellent corrosion resistance of aluminum, a process for aluminum plating could be used advantageously in easing the shortage of more critical plating materials, such as tin, of which there are no important deposits in the United States.

Processes for electrodeposition of aluminum on other metals have been developed in the past. However,many inherent unsolved problems have prevented their use in large-scale operation.

Various electrolytic systems for the deposition of aluminum have been carefully investigated whichinclude organic solutions, aqueous inorganic solutions, non-aqueous inorganic solutions, and molten salt baths. In has been determined that it is not possible to electrodeposit aluminum from aqueous solutions because aluminum has a higher decomposition potential than hydrogen even at high hydrogen overvoltages. Molten salt baths have been employed in producing ductile, adherent deposits of alumiice including cycloaliphatic ether, containing from 4 to 6 carbon atoms, and aluminum chloride.

In general, the electroplating bath is prepared by dissolving a predetermined amount of aluminum chloride in one-half of the predetermined amount of aliphatic ether, and dissolving the predetermined amount of alkylamines in the other half of the aliphatic ether. The resulting solutions are cooled and then mixed. The mixture is then ready for use in a standard electroplating cell.

While it has been found that very satisfactory operation may be carried out in a closed cell at atmospheric pressures, it is not necessary to exclude air from the operation of this cell. However, if a closed cell is used and fitted with some form of cooled reflux condenser, the loss of volatile components of the bath is prevented and the cell may be operated at higher temperatures.

num when operated in a narrow range of compositions and conditions. However, molten baths containing aluminum chloride evolve copious fumes for which adequate means of disposal must be provided. Further disadvantages such as electrolyte degeneration, the high resist ance of the bath and the low optimum current density result in slow and uneconomical deposits of aluminum.

In is a primary object of the present invention to provide a method of electrodeposition of aluminum and alumimun electrodeposition bath which is stable to moisture, inexpensive, and easy to control thereby providing a commercially feasible aluminum plating system.

A further object is to provide a process enabling alumi num to be electroplated on various base metals.

A further object of the present invention is to provide a novel electroplating bath which can be successfully and economically employed for electrodeposition of aluminum on a commercial scale. A still further object of the present invention is to provide a bath for the electrodeposition of aluminum which has a relatively high electrical conductivity, low voltage requirement and in which high electrode eflficiencies are obtainable.

Yet another object is to provide an improved long life electrolyte for depositing aluminum on a metal or other electrically conductive surface.

In general, the bath for depositing the aluminum coating comprises an organic solution comprising alkylarnines containing from one to three alkyl groups having a total of from four to twelve carbon atoms, an aliphatic ether,

It is generally desirable to subject the cell to a period of electrolysis to eliminate impurities such as water and heavy metals at a current density of about 44 amperesrper square foot before electroplating of aluminum is commenced. The temperature of the electroplating bath may be varied over a widerange; for example, satisfactory results have been obtained with the temperature of the bath between ambient room temperature and the boiling point of the particular solution. If higher temperatures are employed in the bath, it is necessary to provide means for condensing escaping vapors and returning the condensate to the bath as hereinbefore described.

It will be seen from the specific examples presented hereinafter that a wide range of mixtures of the alkylamines, aliphatic ethers and aluminum chloride provide satisfactory smooth, bright and adherent aluminum deposits. However, it hasbeen found that most satisfactory results were obtained when the electroplating bath is a single phase liquid mixture. For example, no deposits of aluminum are possible from mixtures which are solids at ordinary temperatures and the deposits of aluminum may be defective because of inclusions of solid particles, if the bath comprises a suspension of solids in liquids.

It has also been found that a wide range of current densities may be employed in the present electroplating procw ess and the use of a periodically reversed direct current has in some instances provided brighter deposits than simple direct current. Y

When steel cathodes are employed in the process of the invention, smooth and bright finishes and a better adherence of the aluminum deposit is provided if the steel is provided with a preliminary deposit of zinc and this preliminary deposit of zinc may also be employed with satisfactory results when plating aluminum on other metals than steel.

The following examples will serve to illustrate the invention with greater particularity.

Example I Example 11 An electrolytic solution is prepared consisting of 31.3 mol percent aluminum chloride, 31.3 mol percent nbutyl amine, and 37.4 mol percent diethyl ether. The solution was placed in a suitable plating tank and satisfactory aluminum deposit on copper and steel was obtained when the bath was maintained at a temperature of from about 27 C. to about 75 C. employing an alunum ano e apdsatho en ti s f o a sw 11 3, to about 2.70 ampereslperlsquare decimeter.

Example lll Aluminum was electrodeposited on a steel cathode from anIelectrolytic bath'consisting of 28.0 mol percent aluminum chloride, 24.2 mol percent n-butyl amine, and 47.8 mol percent diethyl ether, when the temperature of the bath rang'e d tromabout 27 C. .to about 70 C., employing an aluminum anodeandcathode current densities of from about 2.17 to about 2.69 amp'er'es per square decimeter.

; Example IV Aluminum was deposited on a copper cathode from an electrolytic bath consisting of 14.0 mol percent aluminum chloride, 14.5 mol percent n butyl amine, and 71.5 mol percent tetrahydrofuran when the bath was maintained at a temperature offrom about 30 C. to about 75 C., employing an aluminum anode and a cathode current density of about 0.054 ampere per square decimeter.

Example V Aluminum was satisfactorily electrodeposited on a copper cathode from anelectrolytic bath consisting of 15.6 mol percent aluminum chloride, 13.6 mol percent 2-ethylhexyl amine, and. 70.8 mol percent tetrahydrofuran when the bath was maintained at a temperature of from about 50 C. to about 95 C. employing an aluminumanode and a cathode current densityofabout 7.6 amperes per square decimeter.

Example -.VI

An electrolytic bath consisting of 21.5 mol percent aluminum chloride, 14.4 mol percent n-butyl amine, and 64.1 mol per cent n-propyl ether was found to pro vide satisfactory electrodepositions of aluminum on a copper cathode when the bath was maintained at a temperature of from about 25 C. to about 105 C., employing' an aluminum anode and cathode current densities from about 0.78 to about 4.46 amperes per square decimeter.

Example VII An electrolytic bath consisting of 16.0 mol percent aluminum chloride, 9.5 .mol percent 2-ethylhexyl amine, and 74.5 .mol percent n-propyl ether was found to provide satisfactory depositions of aluminum on a copper cathode when the bath was maintained at a temperature rangeof .from about-95 C. to about 105 C., employing an aluminum anode and cathode current densities from about 0.60 to about 1.40 amperes per square decimeter.

r Example VIII An electrolytic bath consisting of 26.4 mol percent aluminum chloride, 18.2 mol percent n-butyl amine, and 55.4 mol percent n-butylethyl ether deposited satisfactory aluminum .on a, copper cathode when maintained in -the temperature range of from about60 C. to about 100 C., employing an aluminum. anodeand .cathode current densities of from about 0.24 toabout 13.574 .amperes per square decimeter.

Example IX Satisfactory deposits of aluminum on a copper cathode is provided from an electrolytic bath consisting of 25.9 mol percent aluminum chloride, 17.2 mol percent 2-ethylhexyl amine, and 56.9 mol percent n-butylethyl ether when the bath-is maintained at a temperature of from about 85 C. to about 105 C., employing an aluminum anodeand cathode current densities of from about 2.65 ,tovabout 4.70 amperes per square decimeter.

4 Example X Aluminum was electrodeposited on a copper cathode from an electroyltic bath consisting of 40.0 mol percent aluminum chloride, 25.0 mol percent 2-ethylhexyl amine, and 35.0 mol percent diethyl ether when maintained at a temperature of from about C. to about 110 C,,,emp10ying an aluminum .anode andcathode current densities in the range from about 1.85 to about 5.26 amperes per square decimeter.

Example XI Aluminum was electrodeposited on a steel cathode from an electrolytic bath consisting of'40.0.mol percent aluminum chloride, 1 25.0 mol percent 2-ethylhexyl amine, and 35.011101 percent diethyl ether when main tained at a temperature of from about 70 C. to about 110 C, employing an aluminum anode and cathode current densities in the range. from about 1.85 tosabout 5.26 amperes per square decimeter.

Example 'XII Aluminum was electrodeposited on a copper cathode with cathode current eifi cie n cies ,of from about 7.5 .to about, 91.0 percent from an: electrolytic bath consisting of 25.6 mol percent aluminum chloride, 15.2 mol percent 2- ethylhexyl aminepand59.2..mol percent diethyl ether when the bath, was maintained at a temperature of from about C. to about C., employing an aluminum anode and cathode current density in v the range of from about 0.43 to,,about 7.78 amperes per square decimeter. 8

Example XIII A deposit of aluminum ,on a gincplatcpcpper cathode from s an electrolytic solution consisting of v25 .6 11101. percent aluminum chloride, 15.2 1110.1 vPfil centZ-etlutlhexyl ami e an 5 mql crs ethyl ther wh n the. bath was maintained ata temperaturefro n about 45 C. to about 60 .C. employingv an aluminurn anode and cathode Mlawitk of f om .l ql fi .-.Z. v abou 2-74amp r w r u r d cim Example- XV A deposit of alumintun on a z ine plated steel cathode from an electrolytic solutionconsistingof 25.6 mol percent aluminum chloride, 15. 2 mol percent Z-ethylhexyl amine, and 59.2 mol percent diethyl ether when the bath was maintained at a tempcrature of from about 45 C. to about 60 C. employing an aluminum anode, and cathode current densities of from about 1:28 toabout 2.74 amperes per square decimeter.

From. the foregoing description and the preceding examples, itwill be seen that the process andelectrolytic solution ,of the .present invention provide means for obtaining,,electrodeposition. of aluminum. .on .base metals such as copp ni r rfit fil and thv zlikeand; theiinvention ,n 7 9 im t the lla i ll illus rat d. embodh merits ,ne einbe cre ,set forth.

' We claims" V l. An alum inum electroplating bat lr comprisinga single phase liquid solution consisting essentiallyof. from about 10 to about 45 mol percent aluminum chloride; from about 10"to about 40 mol percent alkylamine containing from one 'to-three alkyl groups having a total of from four to twelve carbon atoms, and from about 30 to about 80 mol percent of aliphatic ether containing from four to six carbon atoms.

2. The invention defined in claim 1 wherein the alkylarnine comprises n-butyl amine and the aliphatic ether comprises diethyl ether.

3. The invention defined in claim 1 wherein the alkylamine comprises n-butyl amine and the aliphatic ether comprises tetrahydrofuran.

4. The invention defined in claim 1 wherein the alkylamine comprises ethylhexyl amine and the aliphatic ether comprises n-propyl ether.

5. The invention defined in claim 1 wherein the alkylamine comprises 2 ethylhexyl amine and the aliphatic ether comprises n-butyl-ethyl ether.

6. The invention defined in claim 1 wherein the alkylamine comprises 2 ethylhexyl amine and the aliphatic ether comprises diethyl ether.

7. The process of plating aluminum on dissimilar metals by electrodeposition which comprises subjecting, as cathode, a metallic article to be plated, to the action of an electric current while immersed in a single phase liquid solution consisting essentially of from about to about 40 mol percent alkylamine containing from 1 to 3 alkyl groups having a total of from 4 to 12 carbon atoms; from about 30 to about 80 mol percent an aliphatic ether containing from 4 to 6 carbon atoms and from about 10 to about 45 mol percent of aluminum chloride.

8. An aluminum electroplating bath comprising a single phase liquid solution consisting essentially of from about 10 to about 45 mol percent aluminum chloride; from about 10 to about 40 mol percent of a primary alkylamine having a total of from 4 to 12 carbon atoms, and from about to about 80 mol percent of an aliphatic ether containing from 4 to 6 carbon atoms.

9. The process of plating aluminum on dissimilar metals by electrodeposition which comprises subjecting, as cathode, a metallic article to be plated, to the action of an electric current While immersed in a single phase liquid solution consisting essentially of from about 10 to about mol percent of a primary alkylamine having a total of from 4 to 12 carbon atoms; about 30 to about 80 mol percent of an aliphatic ether containing from 4 to 6 carbon atoms and from about 10 to about mol percent of aluminum chloride.

Journal of the Electrochemical Society, vol. 99, No. 6, pages 234-244. 

1. AN ALUMINUM ELECTROPLATING BATH COMPRISING A SINGLE PHASE LIQUID SOLUTION CONSISTIG ESSENTIALLY OF FROM ABOUT 10 TO ABOUT 45 MOL PERCENT ALUMINUM CHLORIDE; FROM ABOUT 10 TO ABOUT 40 MOL PERCENT ALKYLAMINE CONTAINING FROM ONE TO THREE ALKYL GROUPS HAIVNG A TOTAL OF FROM FOUR TO TWELVE CARBON ATOMS, AND FROM ABOUT 30 TO ABOUT 80 MOL PERCENT OF ALIPHATIC ETHER CONTAINING FROM FOUR TO SIX CARBON ATOMS. 